This invention relates to compounds that can be used as nuclear reactor fuels. More particularly, it relates to oxide, once-through pIutonium Fuel compounds that can be used for nuclear Fission in currently operating light-water reactors and fast reactors.
The nuclear fuel compounds that are used in currently operating light-water reactors and Fast reactors are uranium dioxide (UO.sub.2), gadolinia (Gd.sub.2 O.sub.3) doped uranium dioxide, and mixed uranium and plutonium dioxide ((U,Pu)O.sub.2). These nuclear Fuel compounds are pressed into pellets and enclosed in metal cladding, which is made of either a zirconium alloy in light-water reactors or stainless steel in fast reactors. The thus manufactured fuel assembly is used in a nuclear reactor, where it is irradiated with neutrons and part of the uranium and plutonium in the fuel compounds undergoes fission to produce a group of elements that are called "fission products" (FP). The resulting thermal energy is used for electric power generation.
The proportion by which uranium and plutonium in a fuel compound undergoes nuclear fission is called "burnup" and expressed in percent (%). Burnup, which is determined primarily by the operating conditions of a nuclear reactor and the stability of fuel cladding, is in the range from 3 to 5%. In other words, 3-5% of the uranium and plutonium in the fuel compound undergoes nuclear fission. The spent nuclear fuel is dissolved in acid, and the uranium and plutonium which are useful as fuels are separated from fission products. The procedure involving these steps is called "reprocessing". The separated uranium and plutonium are re-converted to a nuclear fuel compound for another use. The fission products are melted in glass and subsequently solidified (vitrified). The solid glass, which is referred to as a high-level radioactive waste, is buried in a deep geological formation. Since the flows of uranium and plutonium form a cycle, the materials flow described above is conventionally referred to as "a nuclear fuel cycle".
With the constant pressure for nuclear disarmament added to the production of plutonium in the nuclear fuel cycle, the excess amount of plutonium has been a global problem and efforts are being made in various countries of the world to develop effective methods that permit plutonium to be used or processed at sites other light-water reactors and fast reactors. The proposals made to date are classified to fall within one of the following two categories (W. J. Broad, Inter. Herald Tribune, Apr. 7, 1993):
(1) Use as a fuel in a new type of reactor
A helium-cooled reactor is newly developed so that it can be operated with plutonium used as a fuel. The problem with this idea is that huge amounts of expenditure and time are necessary to develop the new type of reactor.
(2) Disposal after vitrification
Excess plutonium is simply converted to waste without further use. The processing cost is smaller than in the case of developing the first method; however, from the viewpoint of nuclear fuel cycle which aims at effective utilization of plutonium, the loss of resources is extremely great and, furthermore, the long-term stability of the solid glass is also an important consideration since it has solidified after incorporating a large amount of molten plutonium.